Initial move of canvas/context/snapshot size from u64 -> u32

Changed the two instances of `euclid::default::Size2D<u64>` in
`servo/components/script/canvas_context.rs` and
`servo/components/shared/snapshot/lib.rs` that were outlined as the bare minimum
in "Make canvas/context/snapshot size be u32 everywhere" to
`euclid::default::Size2D<u32>`. Every other change made in this commit is
either:
 - of similar nature, and is the minimum that would allow compilation
 - resolving lints triggered by the former

More might be needed to complete the issue, but I feel like this is a good
starting point.

This commit includes changes to the following components:
 - canvas
 - pixels
 - script
 - shared/canvas
 - shared/snapshot

Signed-off-by: hashcatHitman <155700084+hashcatHitman@users.noreply.github.com>

components/script/canvas_context.rs

@@ -48,7 +48,7 @@ pub(crate) trait CanvasContext {
         true
     }
 
-    fn size(&self) -> Size2D<u64> {
+    fn size(&self) -> Size2D<u32> {
         self.canvas().size()
     }
 
@@ -73,12 +73,12 @@ pub(crate) trait CanvasContext {
 }
 
 pub(crate) trait CanvasHelpers {
-    fn size(&self) -> Size2D<u64>;
+    fn size(&self) -> Size2D<u32>;
     fn canvas(&self) -> Option<&HTMLCanvasElement>;
 }
 
 impl CanvasHelpers for HTMLCanvasElementOrOffscreenCanvas {
-    fn size(&self) -> Size2D<u64> {
+    fn size(&self) -> Size2D<u32> {
         match self {
             HTMLCanvasElementOrOffscreenCanvas::HTMLCanvasElement(canvas) => {
                 canvas.get_size().cast()
@@ -164,7 +164,7 @@ impl CanvasContext for RenderingContext {
         }
     }
 
-    fn size(&self) -> Size2D<u64> {
+    fn size(&self) -> Size2D<u32> {
         match self {
             RenderingContext::Placeholder(context) => (*context.context().unwrap()).size(),
             RenderingContext::Context2d(context) => context.size(),
@@ -251,7 +251,7 @@ impl CanvasContext for OffscreenRenderingContext {
         }
     }
 
-    fn size(&self) -> Size2D<u64> {
+    fn size(&self) -> Size2D<u32> {
         match self {
             OffscreenRenderingContext::Context2d(context) => context.size(),
         }